Fine pitch electrical components are engineered to fit in small spaces on printed circuit boards. One type of fine pitch component is the Quad Flat Pack (QFP), a rectangular component having leads extending from the four sides of the body. QFP's can have 208 leads spaced on a 0.020 inch pitch with 0.010 inch spacing between leads.
Other types are Ball Grid Array (BGA) and Column Grid Array (CGA) components, which have, respectively, a multiplicity of balls or columns on a bottom surface of the body to connect with a printed circuit board.
Because of the very closely spaced leads or connectors, fine pitch electrical components cannot be readily positioned on a printed circuit board by hand. Placement of a fine pitch component is more difficult in rework or repair, when the printed circuit board is already populated with other components or devices. The space between components can be as small as 0.035 inches which makes manipulation of the rework component even more difficult.
Automatic or semiautomatic machines with sophisticated vision systems are available for placing fine pitch components on printed circuit boards. Manual devices using split optic vision systems are also available. Both types of machines are prohibitively expensive for small quantity assembly or repair or rework. In addition, the manual machines do not include solder paste printing capability, and an additional apparatus must be provided for this purpose.
Solder paste screening is the method of choice for providing the proper amount of solder on the printed circuit board. In original assembly work, a stencil is placed over the board and solder is applied to the entire foil pattern at one time. On repair or rework, stenciling is complicated by the components populating the board, which interfere with the placement of the stencil.
The present invention provides a method and an apparatus that overcome the deficiencies in the art. The method and apparatus can be used for low quantity production or prototype work for individual component placement and solder paste application. The invention can also be used for replacement of individual components on a populated printed circuit board in rework or repair.
The apparatus of the invention includes a template that accurately locates the mounting position of the component and the position of a solder paste stencil. The template is made to have substantially the same space footprint as the component which it is used to position and thus can be readily positioned on a populated printed circuit board.
The template includes means for aligning an electrical component, a multiplicity of depressions or holes that receive the pin outs of the component, and means for aligning a stencil on the template, preferably tabs that register with notches on the edges of the stencil.
According to the invention, the template includes a window and shelves formed in a central part for fastening the template to a printed circuit board using tape. The shelves provide a tape adhering location on the template. The window allows for tape adhered to the template to attach to the printed circuit board. By fastening the template in a area within a peripheral outline, the template is readily positionable on a populated board. The shelves are recessed below an upper surface of the template so that tape applied to the template does not interfere with the component being aligned on the template.
According to an alternatively embodiment, the template includes a plate defining diagonally opposite corners of a component pin out pattern connected by a recessed shelf. The template is fixed to a printed circuit board by applying tape across the shelf, within a rectangular outline defined by the template corners. The shelf is recessed to position the tape below an upper surface of the template as in the previous embodiment.
The stencil includes holes for applying solder paste to the printed circuit board, and notches for aligning the stencil with the template.
A pickup device having two heads is used to lift the component and the stencil from the template once they are aligned. According to a preferred embodiment, the pickup device uses vacuum pickup heads. Alternatively, other devices that can lift and hold the component and stencil may be used, for example, a magnetic pickup, a clamping pickup, or pickup using adhesive.